This invention relates to a novel golf ball cover which contains a low cost non-ionic copolymer in combination with an ionic copolymer. More particularly, the present invention relates to a golf ball cover containing at least one ionic copolymer and at least one non-ionic copolymer wherein the latter is selected from the group consisting of a copolymer of ethylene and acrylic acid, a copolymer of ethylene and methacrylic acid, a copolymer of polypropylene and acrylic acid, and blends thereof. The present invention is useful because it provides a high quality golf ball cover at a relatively low cost.
The coefficient of restitution ("C.O.R.") of a golf ball is generally measured by propelling a finished golf ball against a hard surface at a fixed velocity, and measuring its forward velocity (before it rebounds) and rebound velocity (after it rebounds). The ratio of the rebound velocity to the forward velocity is the C.O.R. The C.O.R. must be carefully controlled in all commercial golf balls in order for the ball to be within the 255 ft./second initial velocity standard set by the United States Golfer's Association.
Although the core and the cover both contribute to the C.O.R., this invention addresses the C.O.R. as affected by the cover material.
Golf ball covers having a high C.O.R. and a combination of other beneficial properties, such as high durability, a high cut resistance, and a long life, have traditionally been made from a blend of two or more ionic copolymers in particular proportions. For example, U.S. Pat. No. 3,819,768 (Molitor) discloses a golf ball cover having these mentioned properties comprising a mixture of from 10%-90% of an ionic copolymer of a sodium salt and from 90%-10% of an ionic copolymer of a zinc salt. U.S. Pat. No. 4,333,247 (Ketches et al.) discloses a golf ball cover composed of at least three ionic copolymers.
A problem with these golf ball covers is that they require that 100% of the copolymer in the composition be ionic. Even U.S. Pat. No. 3,454,280 (Harrison), which utilized a single ionic copolymer, required that it constitute 100% of the disclosed covers. However, a disadvantage with utilizing 100% ionic copolymers as the sole source of copolymers in a golf ball cover is that the ionic copolymers are expensive.
Golf ball covers produced from less expensive copolymers of varying types have lacked the durability, the cut resistance, and the high C.O.R. exhibited by golf ball covers containing 100% ionic copolymers.
U.S. Pat. No. 3,454,676 (Busse) teaches the blend of an ionic copolymer with an elastomer to produce a solid one piece golf ball. Although the ionic copolymer/elastomer blend of Busse was suited for producing 1 piece golf balls, such as used on driving ranges, it had an insufficient C.O.R., durability, and cut resistance to be used as the thin skinned cover material in the production of a high quality two-piece or three-piece competition golf ball.
Other compositions which were intended to improve the characteristics of a golf ball cover included mixtures of an ionomer resin either with chlorosulfonated polyethylene (e.g., U.S. Pat. No. 3,940,146) or with an ethylene-vinyl acetate copolymer. However, according to U.S. Pat. No. 4,234,184 (Deleens, et al.), which was filed on Feb. 15, 1979, "the covers prepared from these [two] compositions [among others] were not judged wholly satisfactory."
An object of the present invention is to produce a less costly golf ball cover that does not significantly sacrifice the desirable properties of high durability, high cut resistance, and high C.O.R. (or initial velocity) which are found in golf ball covers employing 100% ionic copolymer.